The National Institute for Occupational Safety and Health (NIOSH) has determined that acetaldehyde and malonaldehyde are potentially carcinogenic to occupationally exposed workers. NIOSH is also concerned about exposure to nine related low-molecular-weight aldehydes: acrolein, butyraldehyde, crotonaldehyde, glutaraldehyde, glyoxal, paraformaldehyde , propiolaldehyde, propionaldehyde, and valeraldehyde. Long-term inhalation studies of acetaldehyde produced laryngeal cancers in hamsters and nasal cancers in rats. A long-term gavage study of malonaldehyde produced adenomas and carcinomas of the thyroid gland and adenomas of the pancreatic islet cells in rats. Acetaldehyde and malonaldehyde have also been shown to be mutagenic in a variety of assays. Adequate epidemiologic data are not available from workers exposed to acetaldehyde or malonaldehyde. However, both chemicals meet the criteria of the Occupational Safety and Administration (OSHA) for potential carcinogens [Title 29 of the Code of Federal Regulations, Part 1990]. NIOSH therefore considers acetaldehyde and malonaldehyde to be potential occupational carcinogens and recommends that worker exposure to acetaldehyde and malonaldehyde be reduced to the lowest feasible concentration. NIOSH is concerned about the nine related aldehydes because their chemical reactivity and mutagenicity are similar to those of acetaldehyde and malonaldehyde, and they may be used as substitutes for formaldehyde, a regulated carcinogen. Although their carcinogenic potential has not been adequately evaluated by in vitro research or studies in experimental animals, careful consideration should be given to reducing occupational exposures to these low-molecular-weight aldehydes.